This invention relates generally to data communications systems and, more particularly, to a digital communication system for transmitting data through a radio frequency (rf) channel that is subject to noise and channel fades.
A wireless rf channel, for use between a mobile station and other mobile stations or a base station, is subject to Rayleigh fading and often gaussian or white noise. An example of such an rf communications channel is a 900 Megahertz (MHz) channel for use in cellular communications.
A 300 baud modem uses asynchronous frequency-shift keying (FSK) to transmit digital data through a communications channel. An asynchronous data communication system typically transmits data in data packets that use a start bit and a stop bit because the exact arrival time of any given data packet is not known. In typical operation of a asynchronous system, a transmitting device holds the communication channel at a predetermined state, for example, one. The channel is then toggled by a start bit, which changes the channel's state (from one to zero). The start bit indicates the beginning of the data packet. The stop bit indicates the end of the data packet and changes the channel back to the wait state. For example, the stop bit toggles the channel data from one to zero and then back to one, if the bit previous to the stop bit was a one, and the stop bit keeps the channel at zero and toggles it to one if the bit previous to the stop bit was a zero.
Data structures having more than one byte of data are sent using protocols that identify the beginning and the end of a data frame using a start-of-frame (SOF) byte and an end-of-frame (EOF) byte, respectively.